A. Field of the Invention
The present invention relates generally to feedback devices and more specifically relates to logarithmic feedback devices for filtering noise from an optical monitoring device.
B. Description of the Prior Art
In certain applications, it is desirable to monitor a small-amplitude, high-frequency signal which is carried by a signal also having a large-amplitude, low-frequency component. For example, laser optic devices for monitoring acoustic signals in ocean systems frequently contain relatively large-amplitude, low-frequency noise due to thermal effects from a laser signal source or related sensing apparatus. Such a system is illustrated in FIG. 1 in which a laser device 6 produces a signal which is split into two components by an optical coupler or a beamsplitter 7 to provide two signals. One signal is simply a reference signal, while the other signal is passed through a modulation means 8 to produce a modulated signal. In such an application, the signal to be detected has an amplitude below that of the random variations in optical power admitted by the typical laser. If an optical coupler were perfect and did not include problems with drift and noise, it would be possible to measure the signal to be monitored by directly compensating the modulated signal which has been acoustically modulated by the parameter to be measured with a reference signal emanating from the same source as the modulated signal prior to modulation. This situation is represented by equation 1 below: ##EQU1##
Where:
V.sub.mod =the voltage of the modulated signal; PA0 V.sub.ref =the voltage of the reference signal; PA0 I.sub.E =the intensity of the optical signal from a laser or other signal source; and PA0 M=the modulation produced in the optical signal by the signal to be monitored. PA0 F.sub.1 =fading induced in the modulated signal primarily due to coupler drift and fiber bending; and PA0 F.sub.2 =fading induced in the reference signal.
As a practical matter, however, optical couplers and the system configuration are not ideal, and they exhibit low-frequency changes during operation. These changes are caused, in part, by thermal fluctuations in the coupler during operation. The effects of these fluctuations, commonly identified as drift, along with laser fluctuations result in what is commonly referred to as a fading channel, may be described as follows: ##EQU2##
where:
In the laser system of the subject invention, the fading factors F.sub.1 and F.sub.2 have relatively large-amplitude effects compared to the modulation factor M. It is therefore impossible to determine changes in amplitude of the modulated factor, M, directly from the output described above. One possible solution to eliminate the relatively large fading effects would be to adjust the gain of both the modulated signal and the reference signal to compensate for fading through the use of an automatic gain control device. This would eliminate the influence of F.sub.1 and F.sub.2 factors in equation 2. Conventional methods to perform this operation are illustrated in FIG. 2. In this system, separate automatic-gain-control (AGC) devices 9 and 11 are used to compensate both the reference signal and the modulated signal to eliminate the fading factors F.sub.1 and F.sub.2. In the system illustrated in FIG. 2, the automatic-gain-control devices 9 and 11 produce the inverse of fading factors F.sub.1 and F.sub.2 such that when the modulated signal is divided by the reference signal through dividing means 13, an output signal of (1-M) is produced. One problem with the system illustrated in FIG. 2 is that each automatic gain control device 9, 11, may introduce its own noise factor into the system. Furthermore, the noise factors produced by automatic gain control devices 9, 11, may be unequal to each other which further complicates interpretation of the output signal.